1. Field of the Invention
The present invention relates to a multi-function lance for use in a vacuum degassing chamber. The invention also relates to a technique for continuously performing, by using such a lance either intermittently or continuously, the multiple operations of preheating a vacuum degassing apparatus (generally comprising a vacuum degassing chamber, an evacuation means, a ladle, etc.) for vacuum refining, composition control and the like, of a molten steel; elevating the temperature of molten steel maintained inside the apparatus; blowing gaseous oxygen for decarburization and refining the molten steel, and blowing a powdered desulfurizing agent or the like for composition control of the molten steel.
2. Description of The Related Art
Recently, the molten steel output from a conversion furnace or an electric furnace is often subjected to further refining (denoted as secondary refining) in order to produce a high quality steel. For such secondary refining, a prevailing method comprises blowing oxygen onto the molten steel maintained inside the vacuum degassing apparatus (which is often referred to hereinafter as xe2x80x9ca degassing chamberxe2x80x9d) to perform decarburization. However, in such decarburization refining, the temperature of the molten steel can be lowered too greatly when the chamber is insufficiently preheated, or smooth operation can be disrupted due to the adhesion of a large quantity of raw metal to the inner wall of the degassing chamber.
Consequently, various countermeasures have been studied and implemented, such as preheating the degassing chamber itself, elevating the temperature of the molten steel, etc. Recently, in JP-A-Hei6-73431 (the term xe2x80x9cJP-Axe2x80x9d as used herein signifies xe2x80x9can unexamined published Japanese patent applicationxe2x80x9d) there was proposed a vacuum degassing apparatus known as a so-called xe2x80x9ccomplex lance 1xe2x80x9d as shown in present FIG. 4, comprising an oxygen blowing portion having a throat portion 15 provided in its axial core, a downwardly extending portion 16 connected to the lower side thereof, and a gaseous fuel supply hole 17 opening into the downwardly extending portion 16. This vacuum degassing apparatus is intended to achieve greatly favorable effects of blowing oxygen to the molten steel, heating the molten steel by burning gaseous fuel with oxygen, and preventing raw metal from adhering to the degassing chamber, etc., by using only one complex lance 1.
However, according to the structure of the complex lance disclosed in JP-A-Hei6-73431, a gaseous fuel is simply blown out from the nozzle portion and mixed with oxygen. Consequently, the calorific value provided thereby was found insufficient as a practical matter to preheat the inside of the degassing chamber. Furthermore, because the structure is extremely simple, the following problems were expected to occur in practice:
(1) The structure is effective only when spontaneous ignition occurs upon mixing the oxygen and gaseous fuel, and the temperature therefore cannot be raised once the temperature inside the degassing chamber has been lowered. Moreover, the reliability is very poor.
(2) Even in case spontaneous ignition occurs, if for any reason the burning gas becomes extinguished during the operation, there remains a danger of causing an explosion due to the fuel gas filling inside the chamber.
(3) Thus, from the viewpoint of safety, the necessary preheating operation of the degassing chamber takes such a long time that productivity is decreased.
The present invention seeks to overcome the above-described problems by providing a multi-function lance for a vacuum degassing apparatus, that is capable of not only performing a preheating operation in a safer manner and in a shorter amount of time, but also blowing oxygen, elevating the temperature of molten steel, heating the vacuum degassing apparatus itself, and refining and controlling the composition of the molten steel by feeding powdered desulfurizing agents and the like, all using the same lance.
In order to achieve this object, the present inventors conducted extensive studies, and as a result developed a multi-function lance for use in a vacuum degassing chamber comprising a water-cooled cylindrical top-blown oxygen lance being equipped with a path for gaseous oxygen, and a nozzle provided at the downstream end of the oxygen path and used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of the top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being positioned in the multi-function lance; and a combustion burners provided at the downstream end of said paths.
In accordance with another aspect of the present invention, there is provided a multi-function lance for vacuum degassing chamber as above, wherein at least one of said burners is equipped with an internally provided ignition plug.
According to still another aspect of the present invention, there is provided a multi-function lance for a vacuum degassing chamber as above, wherein an aperture having a transparent plate is provided axially centrally of the top-blown oxygen lance, and a sensor is provided that detects the flame via the aperture.
According to a further aspect of the present invention, here is provided a multi-function lance for a vacuum egassing chamber wherein a lance for blowing a powder is provided in parallel with the burner as shown in FIG. 5, or, instead of the burners, one or a plurality of lances for blowing a powder, are provided between said top-blown oxygen lance and the water-cooling jacket as shown in FIG. 6.
The present invention also provides a method for using a multi-function lance in a vacuum degassing chamber, comprising actuating a combustion burner disposed in a vacuum degassing chamber comprising a top-blown oxygen lance equipped with a gaseous oxygen path formed with a nozzle provided at the downstream end of said oxygen path used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of the top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being defined between the top-blown oxygen lance and the water-cooled outer jacket; a combustion burners provided at the downstream end of said paths; and a sensor provided as such that it detects the flame via an aperture provided in a transparent plate at the axial center of the top-blown oxygen lance; and supplying a small amount of oxygen to the inside of a top-blown oxygen lance connected to said aperture so as to elevate the temperature of the portion of the flame which is being detected by said sensor.
The invention also includes a method for using a multi-function lance in a vacuum degassing chamber, comprising operating a combustion burner associated with a vacuum degassing chamber that comprises a water-cooled cylindrical top-blown oxygen lance equipped with a gaseous oxygen path formed with a nozzle provided at the downstream end of said oxygen path used for blowing oxygen onto a molten metal; a water-cooled jacket surrounding the outer periphery of said top-blown oxygen lance; one or a plurality of a set of paths for a fluid fuel and a gas for burning the fuel, being positioned in the multi-function lance; and a combustion burners provided at the downstream end of said paths; wherein a powder is supplied to the inside of the top-blown oxygen lance to blow the powder onto the molten metal in the vacuum degassing chamber from the lance.
Thus, the multi-function lance of the present invention prevents the fluid fuel from being extinguished during operation, and enables heating a degassing chamber and blowing of oxygen in a safer and far more stable manner.